The inboard portion of a wind turbine blade is made thick to support thrust and lift loads that are cantilevered onto the blade root by the outboard blade regions. Herein “inboard” means radially inward toward the blade root, which is the portion of the blade connected to the hub. “Outboard” means radially outward, or toward the blade tip. The inboard portion of each blade becomes progressively thicker (perpendicular to the airfoil chord) toward the hub for strength, and typically becomes cylindrical adjacent to the hub to facilitate mounting and interface with a blade pitch adjustment mechanism. The relative air inflow angle changes with distance from the rotation center due to the increasing blade speed relative to the incoming wind. For manufacturing reasons, the chord angle or twist angle of the blade cannot change fast enough near the root to provide the optimal orientation of the blade airfoil section to the relative air inflow direction, resulting in an increasingly excessive angle of attack proximate the root. Due to the blade thickness and high attack angle, the inboard portion of the blade may be permanently stalled, reducing the wind energy conversion efficiency. Herein “attack angle” means the angle between the airfoil chord line and the relative wind vector, taking the blade rotation into account. A stalled condition occurs when air passing over the suction side of the airfoil detaches from the surface of the blade, creating a separated region.
The separated region on the suction side of the blade is slow-moving air, and is accelerated radially outward due to the rotation of the blade. This detached turbulent air is thus pumped outward, and induces or enlarges a separated region on adjacent portions of the blade. To mitigate this outward flow, a barrier known as a stall fence or a boundary layer fence may be placed on the suction side of the blade. Such fences block the radially outward growth and flow of the separated region, thus allowing outboard portions of flow to remain attached longer to the blade, producing more power.